Debris in the coolant which collects or is trapped in fuel rod spacer grids is believed responsible for as many as 30% of known fuel rod failures. Laboratory and in-reactor experience indicate that fuel rod cladding failures can be caused by debris trapped in a grid region which reacts against the fuel rod cladding in a vibratory fashion resulting in rapid wear of the cladding. The size and shape of the debris capable of causing severe damage is quite variable. In fact, metal fragments which can only be picked up with tweezers have been known to "drill" a hole in fuel rod cladding in less than 1,000 hours of reactor operation. Since most failures occur either within or below the first spacer grid, the grids apparently provide a rather good screen for collecting debris. In order to prevent damage in the active area of the reactor, applicants set out to design a device which will trap a large percentage of debris before it gets to the active area.
Previous attempts to treat the debris problem have involved grids in the region of the lower end fitting and the lower rod support grid. One such attempt is the subject of U.S. patent application Ser. No. 020,816 entitled "Debris Catching Strainer Grid", filed Mar. 2, 1987 by Andrew J. Anthony and assigned to the assignee of the instant invention. This grid is typically welded to the upper side of the lower end fitting. Other examples of debris strainers, or traps and grids are seen in U.S. Pat. Nos.: Reissue 27,950; 4,684,495; 4,684,496; 4,652,425; 4,678,627; 4,427,624; 4,096,032; and Japanese Application No. 53-8277, 1-30-1978 (Kokai No. 54-102493, 8-11-1979); German Auslegeschrift No. 1,211,342 (Anmeldetag Feb. 2, 1960); and British Pat. No. 1,214,998. None of these attempts have been totally effective for the purpose intended for reasons of compromise between cost, pressure drop during coolant flow and debris retainability.
Most debris related failures in nuclear reactor fuel are caused by metal shavings, turnings, and pieces of wire that remain in the primary coolant system after maintenance operations. Because of their noncompact shapes, shavings and turnings can be removed from the coolant by passing it through screens having comparatively large holes. More prior art debris interception schemes make use of this technique because it is readily incorporated into existing fuel designs by using a greater number of smaller holes in the lower end fitting.
Debris in the form of wires, having comparatively small cross sections, are more difficult to trap in these devices. Even if these wires strike the debris trap broadside, they have a tendency of bouncing off letting them have another opportunity to penetrate the screen. In order to remove wires from the coolant, flow holes would have to be so small that the pressure drop through them would become unacceptably high, as would their production cost.